1. Field of the Invention
The present invention relates to a method and apparatus for dynamically allocating uplink bandwidth for transmitting frames from an optical network unit (ONU) to an OLT in an Ethernet passive optical network (EPON).
The present invention was supported by the IT R&D program of MIC/IITA. [2005-S-401-02, Project title: High Speed Optical Network Technology].
2. Description of the Related Art
In an EPON, one of optical network technologies, an OLT located at a network side is connected to a plurality of optical network units (ONUs) or optical network terminals (ONTs) located at a subscriber side through optical fibers and splitters, thereby forming a point to multipoint optical network.
FIG. 1 is a diagram illustrating an EPON. In the EPON, an OLT 11 transmits downlink frames to all of ONUs 12 to 14 connected to the OLT 11 in a broadcasting scheme. Herein, the downlink frame is a frame transmitted through a downlink from the OLT 11 to the ONUs 12 to 14. OLT 11 marks the frames with the destination of corresponding frames by inserting a logical link identification (LLID) in a preamble section in the front of the received frame. That is, an ONU having the corresponding LLID is only allowed to receive the downlink frames. Similarly, an uplink frame, which is transmitted from the ONUs to the OLT, also has an LLID in the preamble section thereof in order to inform which ONU transmits the uplink frame.
ONUs 12 to 14 transmit uplink frames to the OLT through the same uplink channel. If more than two ONUs transmit uplink frames at the same time, the OLT 11 cannot normally receive each of the uplink frames because the signals may overlap. Accordingly, the OLT 11 controls the uplink transmission times of the ONUs 12 to 14 and allocates a bandwidth to the ONUs 12 to 14 for transmitting uplink frames. In the EPON, a gate denotes a transmission time allocated to each of ONU. In order to allocate a transmission time to each of the ONU, the OLT 11 transmits a gate frame to the ONU 12. The gate frame includes a grant value that indicates a transmission start time and duration. Hereinafter, terms ‘gate’ and ‘grant’ are used without distinction for convenience.
The ONUs 12 to 14 transmits a request of transmitting uplink frames to the OLT 11 by informing the OLT 11 of the amount of data stored in uplink queues. The request of transmitting uplink frame is referred as a report frame. The ONUs can transmit the report frame in an allocated transmission time. The OLT 11 can dynamically allocate a bandwidth according to the data amount of each ONU, which is transmitted through the report frame.
In the EPON, a multi-point control protocol (MPCP) is used to process auto-registration, reports, and grants. The MPCP may be considered as the extension of a conventional MAC protocol that uses a basic pause.
An OLT transmits all MPCP frames to ONUs with a timer value of the OLT. The ONUs update own timers with the received timer value of the OLT whenever the ONUs receive the timer value of the OLT. The timer of the ONU increases with the same clock of the OLT timer at every moment.
The ONU also transmits the MPCP frame to the OLT with the timer value of the ONU. The OLT calculates a round trip time (RTT) of each ONU by subtracting the received timer value from the own timer whenever the OLT receives the MPCP frame with the ONU's timer value. The OLT uses the calculated RTT of each ONU to transmit the grant frame. In more detail, the OLT decides the start times of grants to ONUs such that gates are not overlapped. When the OLT transmits the gates, the OLT subtracts the RTT value of each ONU from the start time of the grant in order to compensate the transmission delay caused by a distance. According to the compensated start time of the grant, an ONU far away from the OLT transmits an uplink frame early as much as the RTT. Therefore, the uplink frames arrive at the OLT without overlapped regardless of the RTT.
In the EPON system, the duration of the grant is decided at first and the start time of the grant is decided when the gate is decided. In order to effectively communicate, the duration of the grant must be properly allocated to each of the ONUs. The operation of allocation the duration of the grant is the bandwidth allocation operation.
In general, an OLT allocates the bandwidth by regularly collecting reports from ONUs and deciding a gate value of each ONU according to the uplink data amount thereof. Since the OLT does not transmit gate to the ONUs until the OLT receives all of the ONUs' reports, the gate is delayed as much as a dynamic bandwidth allocation cycle in average. Due to the delay of the gate, the transmission delay essentially occurs.
In case of regularly performing a bandwidth allocation operation, overhead increases because additional gate is used for collecting the reports. Therefore, the overall throughput decreases.